Method for removing fine solids from an aqueous bitumen-containing stream

ABSTRACT

The present invention provides a method for removing fine solids from an aqueous bitumen-containing stream, in particular from an aqueous bitumen-containing stream as obtained during an oil sands extraction process, the method at least comprising the steps of:
         (a) providing an aqueous bitumen-containing stream;   (b) subjecting the aqueous bitumen-containing stream to membrane separation using a ceramic membrane, thereby obtaining a bitumen-depleted permeate stream and a bitumen-enriched retentate stream.

This application is a non-provisional application claiming the benefit of U.S. Provisional Application No. 61/716,829, filed Oct. 22, 2012 and U.S. Provisional Application No. 61/705,174 filed Sep. 25, 2012, both of which are incorporated herein by reference.

The present invention relates to a method for removing fine solids from an aqueous bitumen-containing stream, in particular as obtained during an oil sands extraction process.

Oil sands are found in large amounts in many countries throughout the world, but in extremely large quantities in Canada. Such oil sands, also known as bituminous sands or tar sands, contain naturally occurring mixtures of sand, clay minerals, water, and a dense and extremely viscous form of petroleum, technically referred to as bitumen (or also “tar” due to its similar appearance, odour, and colour). Oil sands are mined via open-pit mining and (hot) water is typically used to extract the hydrocarbon content, the bitumen, from these oil sands and the clay minerals. After removal of the bitumen, the bitumen-depleted slurry generally containing various mixtures of coarse solids, sand, silt, clay and water is generally considered oil sand tailings. Because of the presence of fine clay minerals, the produced slurry generally is a suspension that settles slowly. Part of the water is recycled, but a substantial amount is fed into so-called tailing ponds (lakes of fine particles suspended in water) to further settle. As the process consumes a lot of water (up to about 5 volume units of water to produce each volume unit of crude oil) very large tailing ponds have already been created.

Once sufficient settling in the tailing ponds has taken place (which can take a very long time) the supernatant water layer (also called “clear zone tailing water”) may be recycled in the oil sand extraction process. The recycling of the supernatant water layer is hampered by high solids loading and high dissolved calcium (and magnesium) content of the water. The high solids loading and the presence of bitumen in the water causes a lot of problems in the extraction process (such as fouling of heat exchangers), while the high dissolved calcium (and magnesium) content of the water is detrimental to the oil sand extraction efficiency.

The internet showed on 25 Sep. 2012 the Program of the Separation Technology 2012 conference as organised by TEKNA. One presentation (by one of the inventors of the present application) was scheduled for 26 Sep. 2012, had as title “Heavy oil and sand issues” and was to discuss the use of ultra-filtration membrane, dynamic filtration and self-cleaning filtration as alternative technologies to tailing ponds and the extraction of bitumen in an alternative way (not solvents). However, the presentation did not take place.

It is an object of the present invention to solve, minimize or at least reduce the above problems.

It is a further object of the present invention to provide an alternative method for removing fine solids from an aqueous bitumen-containing stream, in particular as obtained during an oil sands extraction process.

It is another object of the present invention to provide a method of reducing the dissolved calcium content of an aqueous bitumen-containing stream.

One or more of the above or other objects may be achieved according to the present invention by providing a method for removing fine solids from an aqueous bitumen-containing stream, in particular as obtained during an oil sands extraction process, the method at least comprising the steps of:

(a) providing an aqueous bitumen-containing stream;

(b) subjecting the aqueous bitumen-containing stream to membrane separation using a ceramic membrane, thereby obtaining a bitumen-depleted permeate stream and a bitumen-enriched retentate stream.

It has surprisingly been found according to the present invention that ceramic membranes efficiently remove fine solids from an aqueous bitumen-containing stream without severe fouling of the membranes. According to the present invention, ‘fine solids’ refer to particles having a particle size of less than 1 μm. Also, it has been found that ceramic membranes efficiently remove dissolved calcium (and magnesium, sodium, potassium) content(s), solids and TOC (Total Organic Carbon) without severe fouling of the membranes. This is contrary to what would have been expected in the field, as membrane technology is regarded as very prone to fouling, in particular when used for hydrocarbon-containing (let alone bitumen-containing) feed streams.

An important advantage of the method according to the present invention is that the removal of bitumen and other undesired components can be achieved in a simple and economical way. This is of particular interest in the processing of an aqueous bitumen-containing stream as obtained from an oil sands extraction process.

A further advantage of the present invention is that no chemical additives (such as flocculants) have to be used for the removal of the fine solids (and reduction of the dissolved calcium content, if the case).

The aqueous bitumen-containing stream is not limited in any way (in terms of composition, phase, etc.), but will typically be an aqueous bitumen-containing stream as obtained in an oil sands extraction process. Alternative sources of the aqueous bitumen-containing stream may be such aqueous streams as obtained in shale gas production, SAGD (Steam-Assisted Gravity Drainage) processes, etc. Examples of aqueous bitumen-containing streams as obtained in an oil sands extraction process that may be suitably processed using the method of the present invention are “clear zone tailing water” (i.e. the supernatant water layer obtained after settling of a tailing pond), “recycle water” (e.g. water originating from the oil sands extraction process, to be used elsewhere in the process), “thickener overflow” (i.e. an aqueous stream containing solids (clay and sand) and relatively high levels of bitumen).

Preferably, the aqueous bitumen-containing stream comprises at least 85 wt. % water, preferably at least 90 wt. %, more preferably at least 95 wt. %, even more preferably at least 97 wt. %. Also, the aqueous bitumen-containing stream preferably has a turbidity of from 20 to 5000 NTU (normal turbidity unit), preferably less than 1500 NTU, as determined according to APHA 2130B (using a HACH 2100N Turbidimeter, as obtainable from Hach Company (Loveland, Colo., USA)).

Usually, the aqueous bitumen-containing stream comprises at least 5 ppm bitumen, typically at least 10 ppm, more typically at least 15 ppm, even more typically at least 20 ppm. Typically, the aqueous bitumen-containing stream comprises at most 500 ppm bitumen. The person skilled in the art will readily understand what is meant by “bitumen” (viz. natural occurring bitumen); hence this is not further explained in detail here. Typically, bitumen is oil having a viscosity greater than 10,000 cP under reservoir conditions and an API gravity of less than 10° API.

Typically, the aqueous bitumen-containing stream has a pH in the range from 3.0 to 9.0, preferably above 5.0 and more preferably above 7.0, and preferably below 8.5, more preferably below 8.0. Also, the aqueous bitumen-containing stream usually has a dissolved calcium content of at least 10 ppm, typically at least 20 ppm as determined according to ASTM D1976-12 (using ICP-AES), and typically less than 100 ppm Ca, typically less than 60 ppm and more typically less than 50 ppm. Further, the aqueous bitumen-containing stream typically contains (again as determined according to ASTM D1976-12):

less than 50 ppm dissolved Mg, typically less than 30 ppm and more typically less than 25 ppm;

less than 1000 ppm dissolved Na, typically less than 800 ppm and more typically less than 500 ppm;

less than 50 ppm dissolved K, typically less than 30 ppm and more typically less than 25 ppm.

Furthermore, the aqueous bitumen-containing stream usually has a Total Organic Carbon (TOC) of at least 10 ppm, typically at least 20 ppm, as determined according to APHA 5310A (whilst using HCl as preservative rather than H₃PO₄ or H₂SO₄). Typically, the aqueous bitumen-containing stream has a TOC of at most 1000 ppm, preferably at most 800 ppm, more preferably at most 500 ppm. This TOC value includes the amount of bitumen present in the aqueous bitumen-containing stream.

The ceramic membrane as used in the membrane separation of step (b) is not limited in any way. As ceramic membranes and processes for the preparation thereof are known, these are not discussed in full detail.

Preferably, the ceramic membrane has a mean pore size of at most 500 nm, preferably at most 100 nm, more preferably at most 50 nm, even more preferably at most 5.0 nm, yet even more preferably at most 1.0 nm. The mean pore size of membranes as used in accordance with the present invention having a mean pore size of above 10 nm is determined according to ASTM F316-03 (2011). The mean pore size of smaller membranes (<10 nm) as used in in accordance with the present invention is determined by Permporometry. Permporometry has been described in Cao, G. Z. et al., “Permporometry study on the size distribution of active pores in porous ceramic membranes.”, Journal of Membrane Science, 1993, 83(2), pages 221-235. Typically, the ceramic membrane has a molecular weight cut-off (MWCO) of from 250 to 10,000,000 Da (g/mol) as determined according to the article of Lee, S. et al., “Determination of membrane pore-size distribution using the fractional rejection of non-ionic and charged macromolecules”, Journal of Membrane Science, 2002, 201, pages 191-201. Preferably, the ceramic membrane has a molecular weight cut-off (MWCO) of below 10,000.

The ceramic membrane may be composed of various inorganic materials, such as alumina, titania, zirconia, silica, SiC, etc. and is preferably composed of TiO₂, ZrO₂, gamma-alumina, SiO₂, or SiC or combinations thereof. Preferably, the ceramic membrane comprises at least 50 wt. % inorganic material, preferably at least 60 wt. %, more preferably at least 70 wt. %, even more preferably at least 80 wt. %, yet even more preferably at least 90 wt. %, or even 100 wt. %. The ceramic membrane may be coated, grafted and/or impregnated.

The ceramic membrane is not limited to any form or size and can be in the form of a monolith, multichannel tubes, hollow fibers, etc. Suitable ceramic membranes are e.g. the Inopor® nano TiO₂ membranes, obtainable from Inopor GmbH (Veilsdorf, Germany); a 1000 kDa membrane obtainable from Orelis Environemt SAS (Salindres, France); and a 1000 kDa membrane obtainable from TAMI Industries (Nyons, France).

The membrane separation of step (b) can be performed in many different ways. Preferably, the aqueous bitumen-containing stream has a temperature during step (b) of at least 0° C., preferably at least 10° C., more preferably at least 15° C., even more preferably at least 20° C. and typically below 100° C., preferably below 90° C. Further it is preferred that the aqueous bitumen-containing stream has a pressure during step (b) of at least 0.5 bara, preferably at least 5 bara, more preferably at least 10 bara and typically at most 50 bara. Typically the Trans Membrane Pressure (TMP) over the ceramic membrane is at least 0.5 bar, preferably at least 1 bar, more preferably at least 2 bar or even as high as at least 5 bar or at least 10 bar.

Also, it is preferred that during step (b) a cross-flow velocity along the surface of the membrane of at least 1.0 m/s is used, preferably at least 1.5 m/s, more preferably at least 2.0 m/s and typically at most 5.0 m/s.

As a result of the membrane separation in step (b), a bitumen-depleted permeate stream and a bitumen-enriched retentate stream is obtained.

Typically, the bitumen-depleted permeate stream comprises at most 20 ppm bitumen, preferably at most 10 ppm, more preferably at most 5 ppm. Preferably, the dissolved calcium content of the permeate stream, as determined according to ASTM D1976-12, is reduced by at least 50%, preferably at least 60%, when compared with the aqueous bitumen-containing stream. Typically, the permeate stream has a dissolved calcium content of below 800 ppm, preferably below 500 ppm, even more preferably below 100 ppm. Also, the permeate stream preferably has a Total Organic Carbon (TOC) of at most 25 ppm, preferably at most 10 ppm, as determined according to APHA 5310A (whilst using HCl as preservative rather than H₃PO₄ or H₂SO₄). Further, the permeate stream preferably has a turbidity of at most 5 NTU (normal turbidity units), preferably at most 1 NTU, as determined according to APHA 2130B (using a HACH 2100N Turbidimeter, as obtainable from Hach Company (Loveland, Colo., USA)). Also, the permeate stream preferably has a recovery of at least 50%, preferably at least 90%, more preferably at least 95% when compared with the volume of the aqueous bitumen-containing stream; if the flow of the aqueous bitumen-containing stream would be 1.0 m³/hr, a 95% recovery for the permeate would result in a flow of 0.95 m³/hr for the permeate and 0.05 m³/hr for the retentate.

Typically the permeate stream is substantially particle-free. The permeate stream will typically be reused as a recycle stream in the oil sands extraction process or sent to a tailings pond; before being used as a recycled stream it may be subjected to several further processing steps, if desired. As a mere example, the permeate stream may be subjected to an ion-exchange resin to remove even more ions, if desired.

Typically, the bitumen-enriched retentate stream comprises at most 1000 ppm bitumen, preferably at most 500 ppm, more preferably at most 200 ppm.

Usually, the bitumen-enriched retentate stream (which is typically solids-enriched) will be disposed of and e.g. combined with a tailings stream as generated elsewhere in the oil sands extraction process or sent to e.g. a filter press.

If desired, the ceramic membrane may be backflushed now and then to remove blockages of the ceramic membrane, e.g., once every 30 minutes. During backflushing, typically an overpressure of at least 1 bar, preferably at least 2 bar, more preferably at least 5 bar, even more preferably at least 6 bar is applied. If needed, the ceramic membrane may be periodically chemically cleaned; however, it has been found that no aggressive chemical cleaning is needed and that mild chemical cleaning once a month suffices, if at all needed.

The person skilled in the art will readily understand that many modifications may be made without departing from the scope of the invention. Further, the person skilled in the art will readily understand that, while the present invention in some instances may have been illustrated making reference to a specific combination of features and measures, many of those features and measures are functionally independent from other features and measures given in the respective embodiment(s) such that they can be equally or similarly applied independently in other embodiments.

The present invention is described below with reference to the following Example, which is not intended to limit the scope of the present invention in any way.

EXAMPLE 1

An aqueous bitumen-containing stream (99.0 wt. % water) as obtained from an oil sands tailings pond was provided as a feed stream. The feed stream was subjected to membrane separation using two M14-19-25-L Inopor® TiO₂ ceramic nanomembrane modules, having a mean pore size of 0.9 nm, obtainable from Inopor GmbH (Veilsdorf, Germany); molecular weight cut-off 450 Da) thereby obtaining a bitumen-depleted permeate and a bitumen-enriched retentate. “M14-19-25-L” refers to 14 multichannels per module, 19 bore multichannels, outer diameter of 25 for the multichannel. As an L (length) of 1200 mm was used, this resulted in 3.51 m²/module.

The following conditions were used during the membrane separation:

-   -   Temperature feed stream: 20° C.     -   Pressure feed stream: 1 bara     -   TMP over ceramic membrane: 10 bar     -   Cross-flow velocity: 2.0 m/s     -   Recovery (of the permeate when compared with the volume of the         feed stream): 50%     -   Backflush cycle: 30 seconds every 15 minutes.

The properties of the aqueous bitumen-containing (feed) stream, the bitumen-depleted permeate and the bitumen-enriched retentate are given in Table 1 below. The permeate appeared as a clean, transparent liquid. The ceramic membrane required mild chemical cleaning (1% citric acid) only once every 10 weeks; further cleaning (with 1% citric acid) was performed when the flux dropped below 10 lmh (litres/m²/hr) at 10 bar TMP.

TABLE 1 Feed stream Permeate Retentate Ca dissolved¹ 21.3 5.0 32.2 [ppm] Mg dissolved¹ 11.7 3.0 17.4 [ppm] K dissolved¹ 16.3 8.8 21.3 [ppm] Na dissolved¹ 324 318 418 [ppm] TOC² [ppm] 43.7 1.5 69.2 Turbidity³ [NTU] 24 0 22 pH 7.50 7.60 7.45 ¹ASTM D1976-12 ²Total Organic Content, according to APHA 5310A (whilst using HCl as preservative rather than H₃PO₄ or H₂SO₄). The bitumen content (not determined separately) was part of this TOC. ³APHA 2130B, (using a HACH 2100N Turbidimeter, as obtainable from Hach Company (Loveland, Colorado, USA))

Discussion

Example 1 surprisingly shows that when a ceramic membrane is used to remove fine solids from an aqueous bitumen-containing stream (such as obtained from an oil sands tailing pond), dissolved calcium content (and content of Mg, K, Na), turbidity, Total Organic Carbon (including bitumen) are effectively reduced without severe fouling of the ceramic membranes. This is a surprising result as membrane technology has typically been regarded as prone to fouling when used for hydrocarbon-containing (let alone bitumen-containing) feed streams, especially when using highly selective membranes for reducing the dissolved calcium content by 50%.

As can be seen, the dissolved calcium content of the permeate stream is reduced by at least 75%, when compared with the aqueous bitumen-containing stream. 

That which is claimed is:
 1. A method for removing fine solids from an aqueous bitumen-containing stream, in particular as obtained during an oil sands extraction process, the method at least comprising the steps of: (a) providing an aqueous bitumen-containing stream; (b) subjecting the aqueous bitumen-containing stream to membrane separation using a ceramic membrane, thereby obtaining a bitumen-depleted permeate stream and a bitumen-enriched retentate stream.
 2. The method according to claim 1, wherein the aqueous bitumen-containing stream comprises at least 85 wt. % water.
 3. The method according to claim 1, wherein the aqueous bitumen-containing stream comprises at least 5 ppm bitumen.
 4. The method according to claim 1, wherein the aqueous bitumen-containing stream has a dissolved calcium content of at least 10 ppm, as determined according to ASTM D1976-12.
 5. The method according to claim 1, wherein the aqueous bitumen-containing stream has a Total Organic Carbon (TOC) of at least 10 ppm.
 6. The method according to claim 1, wherein the ceramic membrane has a mean pore size of at most 500 nm.
 7. The method according to claim 1, wherein the aqueous bitumen-containing stream has a temperature during step (b) of at least 0° C.
 8. The method according to claim 1, wherein the aqueous bitumen-containing stream has a pressure during step (b) of at least 0.5 bara.
 9. The method according to claim 1, wherein during step (b) a cross-flow velocity along the surface of the membrane of at least 1.0 m/s is used.
 10. The method according to claim 1, wherein the dissolved calcium content of the permeate stream, as determined according to ASTM D1976-12, is reduced by at least 50%, when compared with the aqueous bitumen-containing stream.
 11. The method according to claim 1, wherein the permeate stream has a Total Organic Carbon (TOC) of at most 25 ppm, as determined according to APHA 5310A.
 12. The method according to claim 1, wherein the permeate stream has a turbidity of at most 5 NTU (normal turbidity units), as determined according to APHA 2130B.
 13. The method according to claim 1, wherein the permeate stream has a recovery of at least 40%, when compared with the volume of the aqueous bitumen-containing stream. 